This invention relates to a process and system for reducing the concentration of pollutants contained in a gaseous mixture, and more particularly, to a process and system for the removal of particulate matter and the oxides of nitrogen and sulfur from a gaseous mixture, as well as the regeneration and recycling of spent sorbent used to remove the oxides of sulfur from the gaseous mixture.
Particulate matter and the oxides of sulfur and nitrogen result from the combustion or air oxidation of carbon-containing materials, such as coal, fuel oil and the like, and are responsible for major amounts of pollution in our environment. Currently, the commercially available processes and systems for the removal of oxides of sulfur and nitrogen and particulate matter from combustion gases resulting from the combustion or air oxidation of coal or fuel oil in power plants are generally very expensive to build and to operate. The removal of each contaminant requires a large scale system and produces large quantities of waste.
A large number of processes and systems have also been proposed in the literature, including the processes and systems discussed in U.S. Pat. Nos. 3,501,897; 3,776,854; 3,816,597; 3,840,643; 3,966,879; 4,101,634; 4,164,546; 4,170,627; 4,192,855; 4,193,972; 4,258,020; 4,609,537; 4,692,318; 4,744,967 and 4,851,202, all of which are incorporated herein by reference in their entirety. In the foregoing references, either SO.sub.2 is removed; or NO.sub.x is removed; or SO.sub.2 and NO.sub.x as well as particulate matter are removed from combustion gases, usually by using solid sulfur oxide acceptors and/or ammonia gas. In certain instances, it is also known to regenerate the spent or loaded sulfur oxide acceptor by various means as described in U.S. Pat. Nos. 3,501,897; 3,776,854; 3,778,501; 3,846,536; 4,001,376; 4,101,634; 4,164,546; 4,192,855; 4,609,537 and 4,692,318, all of which are incorporated by reference herein in their entirety. In many of the references, the solid sulfur oxide acceptor is used in the form of a moving bed, a fluidized bed, a fixed bed or in a "parallel passage" reactor, and both the removal of sulfur oxides with solid sulfur oxide acceptor and the regeneration of the spent or loaded solid sulfur oxide acceptor are inefficient, inadequate and/or expensive.
In most instances in the prior art, copper, copper oxide or a mixture thereof is coated on alumina or impregnated in alumina to form solid sorbents for the removal of sulfur dioxide from gases. In U.S. Pat. No. 3,966,879, sulfur oxides and particulate matter are removed from waste gases in the same processing zone under the reaction conditions required for sulfur oxide acceptance in a moving bed which contacts the waste gas stream in cross-current fashion.
The reduction of the nitrogen oxides (NO.sub.x), both NO and NO.sub.2, to free nitrogen with ammonia in the presence of a copper oxide-containing catalyst is described in U.S. Pat. No. 4,101,634 where it is also indicated that the conversion of nitrogen oxides with the simultaneous removal of sulfur oxides by means of a copper-containing acceptor proves not to exceed 70 percent.
In U.S. Pat. No. 4,101,634 sulfur oxides and nitrogen oxides are removed simultaneously by use of a metal-containing acceptor with continuous addition of ammonia or precursor thereof with some improvement in efficiency when the metal-containing acceptor is regenerated at regular intervals by a reducing gas passed counter-currently in the bed; the regeneration is terminated at the moment when at least some of the acceptor is still in the sulfate form; and the oxygen-containing gas stream to be purified is then re-contacted with the acceptor with the simultaneous addition of ammonia or precursor.
In U.S. Pat. No. 4,164,546, nitrogen oxides are removed from a gaseous mixture containing nitrogen oxides and oxygen by addition of ammonia thereto and by contacting with a suitable catalyst for the nitrogen-ammonia reaction wherein best results are achieved when sulfur dioxide is also present in the gaseous mixture and wherein excess sulfur dioxide is separated prior to the nitrogen oxide conversion, simultaneously therewith or subsequent thereto. In U.S. Pat. No. 4,164,546, fixed beds of the contact mass, such as copper oxide on alumina are preferred for effective removal of both sulfur dioxide and nitrogen oxide with regeneration by a reducing gas such as hydrogen, methane, ethane, propane and the like. Further, in U.S. Pat. No. 4,164,546, it is indicated that the flue gas may contain small amounts of finely-divided suspended particulate matter such as fly ash.
In U.S. Pat. No. 4,193,972, sulfur dioxides are removed from a gas stream by the use of a metal-containing regenerable acceptor and nitrogen oxides contained in the gas stream are reduced to nitrogen gas in a parallel passage vapor-solids contactor which is conventional for processing gas streams containing particulate matter, such as fly ash, and wherein the reduction of nitrogen oxides to nitrogen is catalyzed by copper sulfate on alumina. In U.S. Pat. No. 4,193,972, the reduction of nitrogen oxides to nitrogen may be carried out simultaneously with the acceptance of sulfur oxides on copper-containing acceptors, and the reaction may be preceded by the admixture of ammonia into the gas stream being treated.
Although regenerative processes wherein spent or loaded solid acceptor used to absorb sulfur dioxide from gas streams is regenerated in processes and systems for the simultaneous removal of sulfur dioxide, NO.sub.x and particulate matter from gas streams are well-known, the processes and systems remain disadvantageous because they are inefficient, require high capital investment when utilized in large scale systems and are expensive to operate in large plants, such as power plants which utilize combustible coal or fuel oil. Even though a certain amount of waste has been eliminated by the foregoing prior art regenerative processes, none of the emerging processes and systems discussed above have gained a wide industrial acceptance due to their high cost and complexity.